The Arabidopsis stearoyl-acyl carrier protein-desaturase family and the contribution of leaf isoforms to oleic acid synthesis
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In plants, changes in the levels of oleic acid (18:1), a major monounsaturated fatty acid (FA), results in the alteration of salicylic acid (SA)- and jasmonic acid (JA)-mediated defense responses. This is evident in the Arabidopsis ssi2/fab2 mutant, which encodes a defective stearoyl-acyl carrier protein-desaturase (S-ACP-DES) and consequently accumulates high levels of stearic acid (18:0) and low levels of 18:1. In addition to SSI2, the Arabidopsis genome encodes six S-ACP-DES-like enzymes, the native expression levels of which are unable to compensate for a loss-of-function mutation in ssi2. The presence of low levels of 18:1 in the fab2 null mutant indicates that one or more S-ACP-DES isozymes contribute to the 18:1 pool. Biochemical assays show that in addition to SSI2, four other isozymes are capable of desaturating 18:0-ACP but with greatly reduced specific activities, which likely explains the inability of these SSI2 isozymes to substitute for a defective ssi2. Lines containing T-DNA insertions in S-ACP-DES1 and S-ACP-DES4 show that they are altered in their lipid profile but contain normal 18:1 levels. However, overexpression of the S-ACP-DES1 isoform in ssi2 plants results in restoration of 18:1 levels and thereby rescues all ssi2-associated phenotypes. Thus, high expression of a low specific activity S-ACP-DES is required to compensate for a mutation in ssi2. Transcript level of S-ACP-DES isoforms is reduced in high 18:1-containing plants. Enzyme activities of the desaturase isoforms in a 5-fold excess of 18:1-ACP show product inhibition of up to 73%. Together these data indicate that 18:1 levels are regulated at both transcriptional and post-translational levels.
KeywordsSSI2/FAB2 Stearoyl-ACP-Desaturase Oleic acid Salicylic acid Jasmonic acid
We would like to thank Amy Crume for help with managing the plant growth facility, John Johnson for help with gas chromatography and Aaron Lewis for help with DNA and FA extractions. We thank David Smith for critical comments on this manuscript. We acknowledge use of the Kansas Lipidomics Research Center Analytical Laboratory and its support from National Science Foundation’s EPSCoR program, under Grant no. EPS-0236913 with matching support from the State of Kansas through Kansas Technology Enterprise Corporation and Kansas State University. We thank ABRC for T-DNA lines. This work was supported by grants from the NSF (MCB#0421914), USDA-NRI (2004-03287) and KSEF (419-RDE-004, 04RDE-006, 820-RDE-007) to AK and PK. JS and EW thank the Office of Basic Energy Research of the United States Department of Energy for support. This study is publication No. 06-12-123 of the Kentucky Agricultural Experiment Station.
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